The invention relates to a method of and apparatus for braking a tractor-trailer combination.
It is well known for agricultural and other tractors to tow trailing vehicles. Numerous examples exist of braked tractor-trailer combinations in many branches of transportation technology. The invention is particularly applicable in the case of an agricultural tractor towing an un-powered trailer such as a grain or beet trailer, hay wagon, baler or similar vehicle that requires the traction of the tractor to effect movement.
The invention is also applicable in other situations in which a trailer is towed. Examples include but are not limited to articulated goods vehicles; combinations of military tractors/cabs and towed vehicles; and two-vehicle forestry and snowmobile trains.
When the tractor or other towing vehicle, as exemplified above, is fitted with four ground-engaging wheels (or other members such as tracks) the invention is useable regardless of whether all four, or just two, of the wheels (or tracks, if present) are driven by the tractor engine.
The foregoing vehicle combinations and types are referred to generally herein as “tractor-trailer combinations”. Furthermore as used herein “trailer” means any of a wide range of ground-supported vehicles that may be towed by a tractor that herein is taken to include any powered vehicle that is capable of towing a trailer.
It is known to provide for automatic operation of the brakes of a trailer. This is desirable for example when driver-actuated brake operation might cause a potential jacknife situation.
A potential jacknife situation arises when the tractor braking power exceeds the braking power of the trailer. In such circumstances the trailer may tend to overtake the tractor with the result that a jacknife of the combination may occur. Automation of the level of trailer braking may help to avoid potential jacknife situations, by removing from the driver of a tractor-trailer combination the need to judge the level of braking that is safe.
One mode of use of the brakes of a trailer is so-called “endurance braking”. This occurs when the brakes of a trailer forming part of a tractor-trailer combination are activated for an extended period without interruption. This may happen for example when the tractor-trailer combination is required to descend a long hill. Endurance braking may be initiated by an automatic process when for example the tractor operator lifts his foot from the engine governor (throttle) pedal (or releases a hand-operated governor) of the tractor at the start of or during a long descent. Software that is associated with the transmission and/or engine control of the tractor then can cause the trailer brakes to be applied automatically so that the tendency of the trailer to overtake the tractor during the descent is controlled.
When endurance braking occurs however the presence of an automated trailer brake valve may lead to a disadvantage. This manifests itself a brake fade (i.e. a gradual reduction in braking power) in the trailer brakes while endurance braking is effected. This phenomenon is caused e.g. by heat build-up in components of the tractor brake such as the brake pads and discs, and results in a reduction in the braking force or effort applied to the wheels or other ground-engaging members of the tractor.
Trailer brake fade of this kind during endurance braking is potentially dangerous because the tractor operator may not be aware that the trailer brakes have activated on releasing of the engine governor as outlined. In consequence the operator may believe he/she has available the full braking power of the tractor-trailer combination yet depression of the tractor brake pedal in an emergency situation may in fact result in only a limited increase in the braking effort applied by the trailer brakes.
The brake fade may be dangerous also partly because the trailer, being inadequately braked, may induce a jacknife situation as outlined above; and potentially because the dynamic performance of the tractor-trailer combination may alter in ways that the tractor operator may not be able to predict.
It therefore is desirable to provide for a more sophisticated control of the braking of an automatically braked trailer during e.g. endurance braking operations.